CN115833949B - Bias method for InP Mach-Zehnder modulator directly coupled to RF driver circuit - Google Patents

Abstract

An optical transmitter includes a directly coupled MZ interferometer and a driver circuit. The MZ interferometer includes a differentially driven MZ electrode pair configured to impart an RF signal to light traveling through a respective arm of the interferometer and receive a DC bias as a positive voltage via a lower n-type cladding of the MZ interferometer. The lower n-type cladding layer is at a positive DC potential different from the upper plane RF ground of the MZ interferometer, but the lower n-type cladding layer and the upper plane RF ground have similar AC potentials. The MZ interferometer further comprises: a series resistor pair configured to provide differential RF termination of the MZ electrodes; and capacitive coupling between a virtual ground formed at a center point between the pair of resistors and an RF ground configured to provide a common mode RF termination. DC power for the driver circuit is applied to the center point of the RF termination.

Description

Bias method for InP Mach-Zehnder modulator directly coupled to RF driver circuit

The present application is a divisional application of chinese patent application with application number 201911310200.6, the application number "bias method for InP mach-zehnder modulator directly coupled to RF driver circuit" at 12/18 of 2019.

Technical Field

The present invention relates to an optical transmitter comprising a directly coupled MZ modulator interferometer and a driver circuit. In particular, the MZ interferometer and the driver circuit can be coupled without a column of RF components between them.

Background

Radio Frequency (RF) amplifier chips are often an important component in optical transmitters that use mach-zehnder (MZ) modulators. The data modulated analog signal output by the Digital Signal Processor (DSP) of the transmitter is typically limited to a peak-to-peak (peak-to-peak) amplitude of less than 1 volt (V). However, the V2 pi characteristic of a typical MZ modulator (e.g., an indium phosphide (InP) based MZ modulator) will typically be about 3V (V2 pi characteristic refers to the voltage required to drive the modulator over a voltage range equal to twice its half-wave voltage (e.g., from-vpi to +vpi), where half-wave voltage is the voltage required to cause a phase shift of pi or a shift from total transmission to maximum extinction).

Without amplification, the signal output by the DSP is therefore too small in amplitude, resulting in MZ underdrive and considerable modulation loss. The purpose of the RF amplifier or driver circuit (driver chip) is to amplify the DSP signal to an amplification compatible with the V2 pi specification of the MZ modulator, thereby minimizing the losses caused by the modulation.

Fig. 1 shows a conventional biasing method in an optical transmitter 10 that includes MZ interferometers 20 and an AC-coupled driver or amplifier circuit 30, it being understood that the transmitter 10 may contain more than one MZ interferometer 20, such as four MZ interferometers in a nested configuration, but in this illustrative example only one MZ interferometer 20 is shown. The configuration of the interferometer will include a modulator chip.

The driver circuit 30 for the transmitter 10 is supplied with a DC power V _DD at the output of the driver circuit 30 through a bias tee (tee) arrangement 40, the bias tee 40 also providing AC coupling of the output of the driver circuit 30 to the MZ interferometer 20 through a column of RF components 50. RF termination (differential and common mode) is provided off-chip as is negative MZ DC bias V _cm. The lower n-cladding of interferometer 20 is strongly coupled to RF ground 70 directly through the entire ground plane and capacitive (-100 pF) chip of the optical subassembly.

In the known arrangement of fig. 1, the driver circuit (driver chip) 30 is thus located remote from the MZ interferometer 20, but on a Printed Circuit Board (PCB) external to the optical subassembly. Such remote "off-chip" locations are advantageous in several respects.

The off-chip location enables the use of a bias tee circuit 40 (for inserting DC power into the AC signal) between the driver circuit 30 and the MZ interferometer 20, with the output of the driver circuit 30 being AC coupled (via a capacitive connection) to the RF signal input pad of the MZ interferometer 20. The AC coupling allows the DC power V _DD of the driver circuit 30 to be applied to the collector output of the driver circuit 30 through a bias tee arrangement, which allows a negative DC bias to be applied to the p-side of the interferometer.

The off-chip location of the driver circuit 30 also allows the differential MZ DC bias V _cm to be applied as a negative voltage to the MZ modulation electrodes 21, 22, typically at the RF termination end 60.

However, the remote location of the driver circuit 30 from the MZ interferometer 20 results in a relatively long RF component column 50, the RF component column 50 also being required to pass through the walls of the MZ interferometer 20 optical subassembly. This can compromise the cascading bandwidth performance of the RF component column 50 due to large RF losses and distortion. It is desirable to provide a device that eliminates these problems.

Disclosure of Invention

In one aspect of the invention, an optical transmitter is provided that includes an MZ interferometer and a driver circuit, wherein the MZ interferometer and the driver circuit are directly coupled. The MZ interferometer includes a pair of differentially driven MZ electrodes configured to impart an RF signal to light traveling through the respective arms of the interferometer and receive a DC bias as a positive voltage via the lower n-type cladding of the MZ interferometer. The lower n-type cladding layer is at a positive DC potential different from the upper plane RF ground of the MZ interferometer, but the lower n-type cladding layer and the upper plane RF ground have similar AC potentials. The MZ interferometer further includes a pair of resistors in series configured to provide differential RF termination of the MZ electrodes; and capacitive coupling between a virtual ground formed at a center point between the pair of resistors and an RF ground configured to provide a common mode RF termination. DC power for the driver circuit is applied to the center point of the RF termination.

Capacitive coupling between the lower n-type cladding layer and the upper planar RF ground may be provided by on-chip (on-chip) capacitive devices and off-chip (off-chip) capacitive devices. The on-chip capacitive device may have a capacitance of about 50pF and/or the off-chip capacitive device may have a capacitance of about 100 nF.

The common mode RF termination may be provided by a capacitive device having a capacitance of about 10 pF.

DC bias can be provided to the lower n-type cladding layer and the peripheral p-type cladding layer of the MZ interferometer. A lower n-type cladding contact may be provided on the upper surface of the MZ interferometer. A lower n-type cladding contact may be provided on the back side of the MZ interferometer.

The MZ interferometer and the driver circuit may be co-located within the optical subassembly of the transmitter, without a bias tee arrangement or RF component column between the MZ interferometer and the driver circuit.

The optical transmitter may include a plurality of MZ interferometers, the MZ interferometers including modulator chips.

In another aspect of the invention, a method of biasing an optical transmitter including a directly coupled MZ interferometer and driver circuit is provided. The method includes applying a positive voltage DC bias to a differentially driven MZ electrode pair of the MZ interferometer via a lower n-type cladding layer of the MZ interferometer, wherein the lower n-type cladding layer is at a positive DC potential different from an upper plane RF ground of the MZ interferometer, but wherein the lower n-type cladding layer and the upper plane RF ground have similar AC potentials. The method further includes providing differential RF termination of the MZ electrodes via a series resistor pair; providing a common mode RF termination via capacitive coupling between a virtual ground formed at a center point between the pair of resistors and an RF ground; the driver circuit DC power is applied at the center point of the radio frequency termination. The method may include providing capacitive coupling between the lower n-type cladding layer and the upper planar RF ground through the on-chip capacitive device and the off-chip capacitive device. The on-chip capacitive device may have a capacitance of about 50pF and/or the off-chip capacitive device may have a capacitance of about 100 nF.

The method may include providing a common mode RF termination by a capacitive device having a capacitance of about 10 pF.

The method can include providing a DC bias to a lower n-type cladding layer and a peripheral p-type cladding layer of the MZ interferometer. A lower n-type cladding contact may be provided on the upper surface of the MZ interferometer. A lower n-type cladding contact may be provided on the back side of the MZ interferometer.

The method may include co-locating the MZ interferometer and the driver circuit within an optical subassembly of the transmitter without a bias tee arrangement or RF component column between the MZ interferometer and the driver circuit.

Drawings

FIG. 1 is a schematic diagram showing a conventional bias in an optical transmitter including a MZ interferometer and an AC-coupled driver circuit;

FIG. 2 is a schematic diagram showing bias in an optical transmitter including a directly coupled MZ interferometer and driver circuit;

FIG. 3 shows a biasing and capacitive coupling scheme for the transmitter of FIG. 2, wherein the electrodes are of the microstrip type;

FIG. 4A is a cross-sectional view along line AA' of FIG. 3;

FIG. 4B is an alternative cross-sectional view along line AA' of FIG. 3;

FIG. 4C is another alternative cross-sectional view along line AA' of FIG. 3;

FIG. 5 shows an alternative biasing and capacitive coupling scheme for the transmitter of FIG. 2, wherein the electrodes are of the segmented type;

FIG. 6 shows an alternative biasing and capacitive coupling scheme for the transmitter of FIG. 2, wherein the electrodes are of the segmented type;

Fig. 7A is a cross-sectional view along line BB' of fig. 6;

Fig. 7B is a cross-sectional view through line CC' of fig. 6; and

Fig. 8 shows the geometry of in-plane (a, b, c) and vertical (d) on-chip resistors.

Detailed Description

An optical transmitter comprising an MZ interferometer and a driver circuit is described herein with reference to fig. 2-8, wherein the MZ interferometer and the driver circuit are directly coupled. A method of biasing an optical transmitter comprising a directly coupled MZ interferometer and driver circuit is also described. For convenience, the following discussion refers to MZ interferometers and driver circuits (driver chips), but it should be understood that these need not be separate entities and may be co-located.

The inventors have recognized that by co-locating the RF driver circuitry with one or more MZ interferometers within the optical subassembly, the cascaded bandwidth performance of the MZ modulator can be significantly improved. In this arrangement, the driver circuit and MZ interferometer are directly coupled, rather than AC coupled. Co-positioning shortens the length of the RF component columns and reduces RF losses and distortion. The absence of an offset tee arrangement also reduces package size.

However, this co-located arrangement requires significant modification to the bias arrangement of the MZ interferometer.

For example, since there is no bias tee arrangement, the driver circuit DC power V _DD can no longer be applied at the driver output. Instead, a driver device (chip) DC power V _DD must be applied to the terminating end of the MZ modulation arm. As a result, the negative MZ DC bias V _cm cannot be applied at the terminating end of the MZ modulation arm, but must be provided as an opposite positive voltage +v _cm to the lower n-type cladding side of the MZ interferometer device.

The RF feed line or waveguide connecting the RF signal input pad to the modulation electrode has a coplanar waveguide (CPW) type and/or a coplanar stripline (CPS) type. The RF signal electric field exists in the horizontal plane between the center signal trace and the peripheral ground trace or balanced differential signal trace, respectively. However, in order to modulate the optical waveguide phase, the RF signal electric field must exist in the vertical plane between the modulating electrode and the underlying n-type lower cladding layer, where the electric field passes through the optical core. Thus, the RF signal electric field must smoothly transition from the horizontal plane to the vertical plane. There must be a sufficiently high speed coupling between the RF ground on the upper plane and the RF ground in the n-type lower cladding layer to achieve this.

In the known arrangement shown in fig. 1, with the AC-coupled driver circuit in the off-chip position, the upper planar RF ground and the n-type lower cladding RF ground are directly coupled, i.e. they are both at the same DC and AC potentials.

However, as described above, in an arrangement where the driver circuit and the one or more MZ interferometers are co-located and directly coupled, the lower n-type cladding layer must be DC biased using a positive voltage +v _cm with respect to RF ground on the upper plane. This arrangement requires capacitive coupling between the lower n-type cladding layer and the RF ground on the upper plane so that they are at similar AC potentials, but different DC potentials. In other words, the lower n-type cladding layer must be at a positive DC potential that is different from the upper planar RF ground, but the n-type cladding layer and upper planar RF ground must be AC coupled or have similar AC potentials to ensure successful transfer of the RF signal to the MZ electrode. The coupling between the upper plane RF ground V _cm and the driver circuit DC power supply V _DD must be at the same RF potential.

As will be described below, this capacitive coupling is provided on-chip and off-chip with low and high capacitance, respectively, thus enabling AC coupling over a wide frequency range.

Fig. 2 shows an optical transmitter 100 in which MZ interferometer 120 and driver circuit 130 (driver chip) are co-located and directly coupled. As previously mentioned, the configuration of the interferometer includes a modulator chip, and as described herein, the driver circuit and modulator chip will be directly coupled. The driver circuit 130 is tightly coupled (i.e., directly coupled) with no bias tee arrangement. Furthermore, there is no RF component column between the driver circuit 130 and the MZ interferometer 120, except for short bonding wires with low inductance.

In contrast to the conventional bias arrangement shown in fig. 1, in the exemplary transmitter 100 of fig. 2, the RF termination 160 between the differentially driven MZ electrodes is provided on-chip. The differential termination is provided as two resistors 161, 162 in series. In this example, each resistor is approximately 50 ohms, providing a total of 100 ohms of differential termination impedance. Or each resistor may provide a resistance of between about 25 and 50 ohms, which in combination provides a total resistance of about 50 to 100 ohms. The Virtual Ground (VG) formed at the center point of the two resistors 161, 162 must be capacitively coupled to the RF ground 170 to provide a common mode termination, thereby ensuring that any common mode excitation of the MZ electrodes 121, 122 is also terminated. Thus, in this example, differential and common mode RF termination is provided on-chip rather than off-chip. Off-chip while on-chip resistors provide a significant improvement in terms of the simplicity of packaging associated with directly coupled driver circuits, alternatively or additionally, resistors may be provided off-chip.

The optimized termination geometry, which is intended to provide a pure resistive impedance, tends to minimize the parasitic capacitance and inductance of the resistors 161, 162. Thus, the resistors 161, 162 are kept short by using high resistive traces and by an isolation stack of implanted (embedded) p-cladding plus intrinsic InP with the resistors 161, 162 overlaid on an RF grounded n-cladding. The optimized resistor geometry will be discussed further below with reference to fig. 8.

In this example, driver circuit DC power V _DD is applied to the center point of on-chip RF termination 160 through inductive bond wire connection L _WB. Typically, a 25 micron diameter gold bond wire will have an inductance of about 0.1 nH/mm. As previously described, MZ negative DC bias V _cm cannot therefore be applied at this location, but is applied to the lower n-type cladding layer as an opposite positive voltage +v _cm, which must be capacitively coupled only to the upper plane RF ground to ensure transfer of signals from the RF feed line to MZ electrodes 121, 122. This capacitive coupling is provided by on-chip and off-chip capacitors 125, 126 (about 50pF and 100nF, respectively, in this example) so that AC coupling can be achieved over a wide frequency range. The common mode termination is provided by a 10pF capacitor 127 in contact with V _cm, V _cm in turn coupled to RF ground 170.

Fig. 3 shows a biasing and capacitive coupling scheme for the optical transmitter of fig. 2, wherein the optical waveguide 150 is split into two branches 151, 152. In this illustrated example, the MZ electrodes 180, 181 of the optical transmitter are of the microstrip type. The differential signals S+ and S-are transferred to the MZ electrodes through GSG CPW RF waveguide W ₁、W₂. The RF signal electric field transitions from the horizontal plane to the vertical plane at the location where the CPW RF waveguide meets the MZ electrode.

In this example, the differential signal s+, S-is differentially terminated by two approximately 50 ohm resistors 161, 162, and the common mode termination is provided by a VG capacitor coupled to MZ DC bias V _cm. MZ DC bias V _cm is in turn capacitively coupled to RF ground, which additionally ensures smooth transfer of differential signal S +, S-onto the microstrip MZ electrode.

In the scheme shown in FIG. 3, a positive MZ DC bias V _cm is provided to the surrounding p-type cladding layer and the lower n-type cladding layer.

Fig. 4A, 4B and 4C are alternative cross-sectional views along line AA' of the arrangement shown in fig. 3, in which MZ DC bias V _cm contacts the peripheral p-type cladding layer in the same manner and the capacitor is in contact with RF ground. The capacitor, which is in contact with RF ground, is formed by a metal layer and a p-type cladding layer with an insulating dielectric layer in between. However, FIGS. 4A, 4B, and 4C illustrate an alternative way in which MZ DC bias V _cm may contact the lower n-type cladding layer.

Fig. 4A illustrates a semi-insulating process in which an n-type cladding contact for a positive DC bias connection must be provided on the top surface of the device (i.e., the upper surface as shown in fig. 4A).

Fig. 4B and 4C show alternatives to the n+ process, wherein contact to the lower n-type cladding layer (and substrate) is provided by a back side (i.e., lower portion as shown in the figures) (fig. 4C) or by both a top side and back side contact (fig. 4B).

Fig. 5 shows an alternative bias and capacitive coupling scheme for the optical transmitter of fig. 2, in this example the MZ electrodes 182, 183 of which are of the segmented traveling wave type. Differential signals S+ and S-are delivered to the segmented MZ electrodes through GSG CPW and CPS RF waveguide W ₃、W₄. The RF signal electric field transitions from the horizontal plane to the vertical plane at the location where the CPS RF waveguide meets the segmented MZ electrode.

The differential signal s+, S-is differentially terminated by two approximately 50 ohm resistors 161, 162 and the common mode termination is provided by a VG capacitor coupled to MZ DC bias V _cm. MZ DC bias V _cm is in turn capacitively coupled to RF ground, which additionally ensures smooth transfer of differential signals onto the segmented MZ electrodes.

As with the scheme shown in fig. 3, the scheme shown in fig. 5 provides a positive MZ DC bias V _cm to the surrounding p-type cladding layer and n-type cladding layer.

Fig. 6 shows another alternative biasing and capacitive coupling scheme for the optical transmitter of fig. 2, in this other illustrative example MZ electrodes 182, 183 are of the segmented traveling wave type. Differential signals S+ and S-are delivered to the segmented MZ electrodes through GSG CPW and CPS RF waveguide W ₅、W₆. The RF signal electric field transitions from the horizontal plane to the vertical plane at the location where the CPS RF waveguide meets the segmented MZ electrode.

The differential signal s+, S-is differentially terminated by two approximately 50 ohm resistors 161, 162 and the common mode termination is provided by a VG capacitor coupled to MZ DC bias V _cm. MZ DC bias V _cm is in turn capacitively coupled to RF ground, which additionally ensures smooth transfer of differential signals onto the segmented MZ electrodes.

In the alternative of fig. 6, all capacitors are provided on the lower n-type cladding layer, as compared to the first bias and capacitive coupling scheme shown in fig. 3 and the second bias and capacitive coupling scheme shown in fig. 5, respectively. This eliminates the need to provide MZ DC bias V _cm to the peripheral p-type cladding layer, which can be more space-saving considering the total chip area.

Fig. 7A and 7B are cross-sectional views along lines BB "and CC' respectively of the arrangement shown in fig. 6, unlike the cross-sectional views of 4A, 4B and 4C described above, the peripheral p-type cladding layer is not provided with a positive MZ DC bias V _cm. In contrast, in fig. 7A and 7B, the required capacitive coupling is performed entirely on the large exposed n-type cladding layer located in the large n-well.

As described above (with reference to fig. 3,5, and 6), the optical transmitter of fig. 2 may use alternative bias and capacitive coupling schemes in which a positive MZ DC bias V _cm is provided to either the peripheral p-type cladding layer and n-type cladding layer (fig. 3, 4, and 5), or to the n-type cladding layer only (fig. 6 and 7).

As described above with reference to fig. 2, 3, 5 and 6, the differential termination of the signal S +, S-is provided by two resistors approximately 50 ohms on a chip in series. Fig. 8A to 8D show three alternative geometries of these on-chip 50 ohm resistors 161, 162.

As described above, these three geometries are typically optimized by keeping resistors 161, 162 narrow and short, by using highly resistive traces, and by overlaying resistors 161, 162 on an isolation stack of an injected p-cladding plus intrinsic optical waveguide core on an n-cladding that is RF grounded.

Fig. 8 shows various geometries suitable for the on-chip geometry of resistors 160, 161. Fig. 8a to 8c show in-plane descriptions of possible geometries, and fig. 8d shows two possible perpendicular descriptions of the geometry of fig. 8 a.

In the in-plane, on-chip resistor geometry shown in fig. 8a, the RF differential lines are terminated using two approximately 50 ohm metal traces connected to a single capacitor at a virtual RF ground. The geometry in the plane is chosen to minimize the length of the resistor to reduce parasitic inductance and capacitance. Thus, narrow and highly resistive metal trace traces are preferred. An alternative in-plane configuration is shown in fig. 8b and 8 c.

Figures 8a and 8b naturally provide point contacts for an equipotential single resistor, while the geometry of figure 8c relies on good ground uniformity across the capacitor and low capacitor resistance between the contact points of the resistors to achieve the same performance. As described above, V _DD is applied at VG point by an induction bond wire connection. Since there is no RF ground connection nearby, VG is capacitively coupled to V _cm,V_cm, which in turn is capacitively coupled to RF ground, as illustrated in FIG. 2.

Fig. 8d shows a vertical arrangement in which parasitic capacitance is minimized by the thick neutral implanted p-layer under the resistor metal trace. Fig. 8d shows an N-doped and Si (silicon) substrate using the same implanted p-layer arrangement. In the case of a Si substrate, the back metal layer is optional.

Each resistor geometry shown in fig. 8 may be used with the transmitter of fig. 2 and with the biasing and coupling schemes described above with reference to fig. 3-7.

Those skilled in the art will appreciate that various modifications might be made to the above-described embodiments without departing from the scope of the present invention.

The optical transmitter includes one or more MZ interferometers, each MZ interferometer including a differentially driven MZ electrode pair, a series resistor pair configured to provide differential RF termination of the MZ electrodes; and capacitive coupling between a virtual ground formed at a center point between the pair of resistors and an RF ground configured to provide a common mode RF termination.

Claims (18)

1. An MZ interferometer, comprising:

a first coupling configured to receive a DC power supply;

a differentially driven MZ electrode pair configured to:

imparting an RF signal to light traveling through a respective arm of the MZ interferometer, an

Receiving a DC bias as a positive voltage via a lower n-type cladding layer of the MZ interferometer;

The lower n-type cladding layer is at a positive DC potential different from an upper plane RF ground of the MZ interferometer and has the same AC potential as the upper plane RF ground;

a second coupling configured to transmit the DC power received from the first coupling through the differentially driven MZ electrode pair;

A resistor pair providing differential RF termination for said differential driven MZ electrode pair; and

Capacitive coupling between the virtual ground and the radio frequency ground,

The virtual ground is formed at a center point between the resistor pairs.

2. The MZ interferometer of claim 1, wherein said capacitive coupling is configured to:

providing common mode RF termination.

3. The MZ interferometer of claim 2, wherein said capacitive coupling is provided by a capacitive device of 10pF capacitance.

4. The MZ interferometer of claim 1, wherein said DC power is applied to said center point by an inductive wire bond connection.

5. The MZ interferometer of claim 1, wherein said capacitive coupling is provided between said lower n-type cladding layer and said upper planar RF ground by an on-chip capacitive device and an off-chip capacitive device.

6. The MZ interferometer of claim 1, wherein said pair of resistors is configured to be connected in series.

7. The MZ interferometer of claim 1, wherein said pair of resistors provides a differential termination impedance between 50 and 100 ohms.

8. The MZ interferometer of claim 1, wherein said second coupling is further configured to directly couple said MZ interferometer and a drive circuit.

9. The MZ interferometer of claim 1, wherein said differential RF termination is provided on-chip.

10. The MZ interferometer of claim 1, wherein said pair of differentially driven MZ electrodes are microstrip-type MZ electrodes.

11. A method, comprising:

Applying a DC power supply to the first coupling of the MZ interferometer;

imparting RF signals to light traveling through respective arms of the MZ interferometer via differentially driven MZ electrode pairs of the MZ interferometer;

applying a DC bias as a positive voltage to the differentially driven MZ electrode pair of the MZ interferometer via a lower n-type cladding layer of the MZ interferometer;

The lower n-type cladding layer is at a positive DC potential different from an upper plane RF ground of the MZ interferometer and has the same AC potential as the upper plane RF ground; and

Transmitting, by a second coupling of the MZ interferometer, the DC power received from the first coupling through the differentially driven MZ electrode pair;

providing a common mode RF termination by capacitive coupling;

The capacitive coupling is located between virtual ground and radio frequency ground; and

The virtual ground is formed at a center point between a pair of resistors of the MZ interferometer.

12. The method of claim 11, further comprising:

Differential RF termination is provided to the differentially driven MZ electrode pair by a resistor pair of the MZ interferometer.

13. The method of claim 11, wherein the applying DC power comprises:

The DC power is applied to the center point by inductive bond wires.

14. The method of claim 11, wherein the capacitive coupling is provided by an on-chip capacitive device off-chip capacitive device between the lower n-type cladding layer and the upper planar RF ground.

15. The method of claim 11, wherein the pair of resistors is configured to be connected in series.

16. The method of claim 11, further comprising:

The MZ interferometer and the drive circuit are directly coupled by the second coupling.

17. The method of claim 12, wherein the differential RF termination is provided on-chip.

18. The method of claim 11, wherein the differentially driven MZ electrode pair is a microstrip-type MZ electrode.